To me the foam, which I'm pretty sure is high density stuff. Like you can jump on it and it deforms only slightly. It would act as a leveling device, which is how/why you were using it in the 1st place, as long as the edges dont dig in so much to be pushing up on the bottom. I assume that since the middle braces look like thin metal and from what little I've read/seen that they only support sides( I know the top one does, just assuming the bottom brace is the same/similar). So in fact the bottom glass is supporting all of the weight, you would think that the seems would leak if it was not level before it would crack or break.
You may never know what exactly caused it, maybe if you looked closely along the crack with a magnifying glass, you could possibly see some small nicks near the "fault line" (crack) that are not part of the actual crack. Then you could assume this was from some impact and what caused the stress(at some time previously). Alternatively if the crack did start at the edge, then I guess not being level could be the cause.
Did it recently have water before you got it? How old was the tank?
Any way, I think it was more of a random incident, and your methods seem sound to me.